1. Field of the Invention
The present invention relates to a differential amplifier suitable for detecting an output current of a zero-phase current transformer in an earth leakage breaker and to an earth leakage breaker formed with the differential amplifier provided.
2. Background Art
FIG. 5 is a circuit diagram schematically showing an example of a typical configuration of an earth leakage breaker. As is shown in FIG. 5, an earth leakage breaker 1 is provided with an interrupter switch 2, a zero-phase current transformer (ZCT) 3, a differential amplifier 4 and a control unit 5. The interrupter switch 2 is inserted in an AC power supply line into which an AC power supply voltage Vac is supplied through a transformer T. The zero-phase current transformer 3 is further inserted in the AC power supply output line into which the interrupter switch 2 is inserted. The differential amplifier 4 amplifies a current detected through the zero-phase current transformer 3. The control unit 5 monitors the output voltage of the differential amplifier 4 to detect ground leakage on the AC power supply output line side. Namely, the control unit 5, when the monitored output voltage of the differential amplifier 4 becomes high (when the output current of the zero-phase current transformer 3 becomes large), determines that earth leakage occurs on the AC power supply output line side to drive the interrupter switch 2 to thereby interrupt the input of the AC power supply voltage Vac (see JP-A-2012-246736, for example).
FIG. 6 is a circuit diagram schematically showing an example of the configuration of the related typical differential amplifier 4 used in the earth leakage breaker 1. The differential amplifier 4 is formed with a differential circuit section A and a gain circuit section B provided. The differential circuit section A carries out the differential amplification of voltages Vin1 and Vin2 applied to a pair of voltage input terminals, respectively. The gain circuit section B amplifies the output of the differential circuit section A and outputs an amplified output voltage OPout with a specified voltage level. Incidentally, the differential circuit section A is formed of a first transistor M1 and a second transistor M2 forming a first differential pair, a third transistor M3 forming a current source of the first differential pair, and a fourth transistor M4 and a fifth transistor M5 forming a current mirror circuit to be active loads of the first differential pair.
Specifically, each of the first transistor M1, second transistor M2 and third transistor M3 is formed of, for example, a p-channel MOS-FET (hereinafter abbreviated as P-MOS). Each of the fourth transistor M4 and fifth transistor M5 is formed of, for example, an n-channel MOS-FET (hereinafter abbreviated as N-MOS). The third transistor M3 is operated with the gate voltage thereof being applied by a tenth transistor (P-MOS) M10 driven by a constant current source Ibias. The third transistor M3 plays a role of supplying a constant tail current Iss to the first differential pair of the transistor M1 and transistor M2.
The gain circuit section B is formed of a transistor (N-MOS) M12 in common-source connection. The transistor M12, with a transistor (P-MOS) M11 as a load, for example, connected to the drain thereof, has a voltage, produced at the drain of the transistor M1 forming the first differential pair, inputted to the gate thereof and carries out inverted amplification of the inputted voltage. The transistor M11, with the gate voltage thereof applied by the tenth transistor M10, operates as the load of the transistor M12. The differential amplifier 4 with such a configuration is as is presented in detail in JP-A-2012-244558, for example.    Patent Document 1: JP-A-2012-246736    Patent Document 2: JP-A-2012-244558
Incidentally, from the view point of preventive maintenance and stability in supply of power, the development of an earth leakage breaker 1 is being carried out which is provided with functions of detecting a state of exhibiting any sign preceding earth leakage, a state in which the change in the level of the output current of the zero-phase current transformer 3 becomes 30% of the earth leakage level, for example, and giving an alarm to attract attention. In the earth leakage breaker 1 of this kind, it is required that not only is the detection accuracy in the zero-phase current transformer 3 improved, but the sensitivity (input detection sensitivity) of the differential amplifier 4 is increased to three times or more, for example.
However, in the related differential amplifier 4 with the configuration shown in FIG. 6, an input offset voltage ΔVin, which is a difference between the voltages Vin1 and Vin2 remaining across a pair of the voltage input terminals when the output voltage OPout is made to be zero, is generally on the order of 10 mV max. This is larger than the detected voltage (output voltage) of the zero-phase current transformer 3 in the state of change in 30% of the earth leakage level. Incidentally, for detecting the detected voltage (output voltage) of the zero-phase current transformer 3 in the state of change in 30% of the earth leakage level, it is necessary to reduce the input offset voltage ΔVin in the related differential amplifier 4 to the order of 2 mV max, for example. Thus, there is a problem in that in the earth leakage breaker 1 formed by using the related differential amplifier 4, it is difficult to detect the state of change in 30% of the earth leakage level explained in the foregoing.
The invention was made with such a situation taken into consideration and it is an object of the invention to provide a differential amplifier which is capable of detecting the state of change in 30% of the earth leakage level with high accuracy and an earth leakage breaker formed by using the differential amplifier and excellent in preventive maintenance and stability in supply of power.